1. Field of the Invention
The present invention relates to a novel palladium (Pd) powder, and, more particularly, to a novel Pd powder which contains alkaline earth metal elements in specific amounts and is useful as a conductive component for thick-film conductive pastes. The present invention further relates to a thick-film conductive paste prepared from such a Pd powder and a multilayered ceramic capacitor having internal electrodes therein which are formed from the conductive paste. The present invention also relates to a production process of the foregoing Pd powder containing alkaline earth metal elements in specific amounts.
2. Description of the Prior Art
A multilayered ceramic capacitor has heretofore been produced by printing a thick-film conductive paste onto ceramic green sheets to form internal electrode patterns thereon, laminating the resultant sheets and firing the laminated sheets. Pd powder has extensively been used in the conductive paste, since the internal electrode patterns formed from the conductive paste are subjected to firing at a temperature as high as 1000.degree. C. or above. However, usually, Pd powder begins to oxidise and expand at about 500.degree. C., which is lower than the sintering temperature of ordinary ceramics, reaches its maximum oxidation rate at about 800.degree. C. and, thereafter, is rapidly reduced to return to the original Pd at 850.degree. C. In particular, fine Pd powder has a high oxidation activity and oxidizes almost completely to cause an increase in its weight by about 15%. Since the ceramic material is poor in strength due to insufficient sintering at a temperature as low as 600.degree. to 700.degree. C., the oxidation and reduction of the internal electrodes have readily caused such structural defects as cracks and delamination.
Various attempts have been made to prevent the generation of such structural defects by the regulation of the particle size of the metallic powder, oxidation treatment or surface treatment of the powder, compounding of additives, and the like. However, an urgent requirement in recent years for size reduction and capacitance enhancement of capacitors has resulted in decreases in the thickness of the ceramic layer and the internal electrode film and in the areas thereof and an increase in the number of laminated layers, thus making it difficult to improve the above-mentioned defects, especially the cracks, by conventional methods.